The present invention relates to device and method for measurement, and more particularly to device and method for measuring vibration.
Conventionally, sensors include two primary categories such as the point sensor and the distributed sensor. For the former, it usually has bandwidth limitation as a result of its structural frequency response. The latter, such as the one disclosed in U.S. Pat. No. 4,868,447, includes the mode sensor and is closely related to the measured structural body.
Since the measurement of acceleration or acceleration rate is a very important information in vibration of the structure system, the accelerometer is directly put into measuring the acceleration at present. Accelerometers normally arc of piezoelectric type or capacitance type. The former relates to an interaction between mechanical energy and electrical energy, which was found in 1880 by Curie brothers, i.e. Pierre Curie and Jacques Curie upon studying the relation between pyroelectric phenomenon and crystal symmetry. (Cady, 1964)
Curie brothers found that when the tourmaline is applied with stress, one can obtain the charge from the surface thereof. They subsequently found such phenomenon in a series of materials, e.g. zinc blende, calamine, boracite, sodium chlorate, quartz and Rochelle salt. The electrical polarization phenomenon produced with the deformation of such materials is called piezoelectricity.
Common piezoelectrical materials generally have three categories, i.e. natural crystal, piezoelectric ceramics and piezoelectric polymer. Since it is very difficult to make a distributed sensor through a natural crystal, e.g. quartz, only piezoelectric ceramics and piezoelectric polymer will be discussed here. Because piezoelectric ceramics, e.g. lead zirconate titanate (PZT) has higher coupling factor and dielectric constant, it is suitable for use in the driving device requiring a large driving force.
Current methods for measuring the acceleration rate are generally achieved by modifying the interface circuit of the accelerometer as disclosed in U.S. Pat. No. 5,521,772. The system response and the bandwidth of the conventional point sensor, no matter whether piezoelectric accelerometers or capacitance accelerometers are concerned, are limited by the frequency response of the sensor structure. In other words, the influence of the electronic circuit on the performance, the bandwidth, gain and phase angle of the conventional accelerometer are not only influenced by the interface electronic circuit but also primarily controlled by structural design of the sensor. General basic requirements of an excellent accelerometer are as follows: high electromechanical conversion efficiency, larger dynamic range, broader bandwidth response, higher stability, light weight, low transverse sensitivity and low environmental sensitivities in respect of factors including temperature, humidity and electromagnetic interference. It is not easy for the point sensor mentioned above, which is subjected to limitations of structural characteristics of its matching sensor, to meet with all design requirements at the same time.
To take the piezoelectric accelerometer as an example, after the piezoelectric material is applied with the accelerometer since 1960, the piezoelectric accelerometer has been extensively put into use in various fields. Generally speaking, apparent advantages of adopting the piezoelectric material as the sensing element of accelerometer include: light weight, small bulk, high reliability and self generation without external power supply necessary for the capacitance accelerometer. The bandwidth limitation of the accelerometer is primarily dominated by the sensor structure. Accelerometers using the piezoelectric material as the sensing element have the following featured designs:
1) Compression design: As shown in FIG. 1, the base 11, i.e. the sensor structure, supporting thereon the piezoelectric material 12, i.e. the sensing element, is connected to the external cover 13 to generate the acceleration signal through compressed deformation of the piezoelectric material;
2) Cantilever design: As shown in FIG. 2, the base 21 serves as the fixed end of the cantilever beam 22 formed by the piezoelectric material, whose free end is fixed to a mass 23 for increasing the acceleration response. The acceleration signal is measured through the strain generated by vibration of the cantilever beam 22;
3) Shear design: As shown in FIG. 3, it includes a base 31, a piezoelectric material 32 and an added mass 33. The signal is generated through the shear strain of the piezoelectric material;
4) Single ended compression design: As shown in FIG. 4, the piezoelectric material 42 on the base 41 is stacked with a mass 43 to be independent of the external cover 44 for dealing with the influence of the sonic vibration;
5) Mushroom design: As shown in FIG. 5, the base 51 attaches thereon a beam 52 which has two free ends and is coated thereon with a piezoelectric material 53 serving as the sensing element.
Basic structural designs of all the above piezoelectric accelerometers are point sensors which measure the acceleration by the electronic signal generated when the strain of the piezoelectric material is subjected to change. No matter whichever sensor design is concerned, purposes of superior dynamic response and broadened system bandwidth are always sought. The bottleneck of the conventional point accelerometer is that the integral system bandwidth of the accelerometer is limited by the resonant mode of the sensor structure. If a better bandwidth is sought by raising the first resonant mode frequency, the accelerometer will have a poor dynamic response to the low frequency. If the structural stiffness of the accelerometer structure is reduced to increase the dynamic response to lower frequencies, the bandwidth of the accelerometer will be lowered.
Alternatively, if the acceleration signal of a specific frequency is to be measured, the conventional method is to provide a filter with the electronic circuit for attenuating the accelerometer signal of undesirable bandwidths. Since this filter modulates signal on the time domain, it must follow the causality. According to the Bode Gain Phase Theorem, phase change of the original acceleration signal will be introduced so that there will be an extreme difference between the measured acceleration signal and the real acceleration signal, which requires an additional compensation to calibrate the measured signal. It is not wonderful in use.
It is therefore tried by the Applicant to deal with the above situations encountered in the prior art.
It is therefore an object of the present invention to provide a vibration measuring device for accurate measurement in a specific bandwidth.
It is further an object of the present invention to provide a vibration measuring device for selective accurate measurement in a frequency range.
It is further an object of the present invention to provide a vibration measuring device utilizing a feedback control loop to moderate the deformation of sensing structure.
It is still an object of the present invention to provide an accelerometer having an increased bandwidth and a reducted bulk.
It is additional an object of the present invention to provide a point distributed piezoelectric sensor by gathering together advantages of distributed sensor and point sensor.
It is yet an object of the present invention to provide a sensor of spatial filtering function having the modulated gain without the influence on the phase angle.
It is furthermore an object of the present invention to provide a spatial filter which will not result in any unnecessary phase lag but has a freely selective bandwidth.
It is again an object of the present invention to provide a modal sensor which can lift the bandwidth limitation resulting from the sensing structure resonance through the manufacture of the higher modal sensor and raising the bandwidth to an even higher mode.
According to an aspect of the present invention, a vibration measuring device for measuring a vibration of an object includes a sensing body generating a dynamic information in response to the vibration, and a piezoelectric sensing layer disposed on the sensing body to construct thereby a sensor and formed in a distributed mode to measure the dynamic information in a selected specific bandwidth.
Certainly, in the vibration measuring device, the sensor can be a point sensor, the object is one selected from a group consisting of building structure, sound equipment and military mechanics, the sensing body is a sensor structure body, and the piezoelectric layer has an effective piezoelectric profile.
Certainly, the present vibration measuring device can be an accelerometer, a device for metering acceleration rate or a force sensor. The accelerometer, metering device and force sensor can respectively have sensor structure bodies respectively having different dynamic informations.
Preferably the sensing layer includes two piezoelectric sheets oppositely adhered with each other and respectively having two piezoelectric surfaces for sensing in the specific bandwidth, in which the two piezoelectric surfaces respectively adhere thereto two surface electrodes collectively forming an effective surface electrode for generating the effective piezoelectric profile.
Preferably the two piezoelectric sheets are oppositely polarized in the effective surface electrode and have an electromagnetic interference shielding effect through a pseudo ground.
Preferably the sensing layer has polarized profile and direction through a potential difference. The potential difference of the sensing layer results in a sensing charge along a thickness of the sensing layer, and the sensing charge being a charge signal in the effective surface electrode is to be measured by the effective surface electrode.
Preferably the effective surface electrode has a specific shape determined by a surface electrode function for determining the specific shape in response to the respective sensing body. The surface electrode function is a weighting function, and the charge signal is presented by a frequency spectrum and modulated by the weighting function. The surface electrode function cooperates with a Heaviside step function for reducing a leaking phenomenon of the frequency spectrum.
Preferably the measuring device is a sensor system, the vibration of the object is revealed by a number of waves in the space domain, and an integration of the surface electrode in the space domain can overcome a phase delay phenomenon of the space waves resulting from a modulated gain of the sensor system in time domain. The phase angle of the space waves is kept constant for simultaneously obtaining past, present and future information of the space waves in order that the measuring device serves as a spatial filter of broad bandwidth or selective bandwidth. The surface electrode function is a Laplace transform function for selecting a filtering effect and facilitating a connection with a control loop.
Preferably the piezoelectric sensing layer is a mode 1 sensor being a matching filter of the sensing body. The present vibration measuring device preferably further includes a second sensor for a broader bandwidth. The second sensor is determined by a second surface electrode function for eliminating a sensitivity with respect to the mode 1 sensor and the second surface electrode function is obtained through expansion from an eigen function so that the second sensor is an anti-mode 1 sensor. The anti-mode 1 sensor has a surface electrode having an inverse polarity with respect to the mode 1 sensor, and has a vertically symmetrical shape for increasing a profile of the charge signal. The anti-mode 1 sensor can cancel an error signal resulting from twisting if the measuring device has a undesired resonant frequency, and the sensing body is a flexible structure for reducing a bulk thereof and raising a low frequency response effect thereof. The anti-mode 1 sensor can be manufactured by a semiconductor procedure, and the measuring device can be a point distributed sensor serving as a low frequency accelerometer.
Generally the effective piezoelectric profile of the piezoelectric sensing layer can vary with a polarization modulation of the piezoelectric sensing layer made of piezoelectric polymer of polyvinylidene fluoride (PVDF), lead zirconate titanate (PZT) or zinc oxide (ZnO).
The selected specific bandwidth can be a frequency domain of low-pass, high-pass, band-pass or band-stop. The present vibration measuring device further includes a second piezoelectric sensing layer for measuring the dynamic information in a second selected bandwidth.
Preferably the present measuring device is an accelerometer, the piezoelectric sensing layer senses the dynamic information to generate an electric charge or a voltage signal and is electrically connected to an interface circuit, and the interface circuit is provided with an amplifier for amplifying the electric charge or the voltage signal to obtain an acceleration signal.
Alternatively, the present vibration measuring device can be a device for metering an acceleration rate, the piezoelectric sensing layer senses the dynamic information to generate a current signal and is electrically connected to an interface circuit, and the interface circuit is provided with an amplifier for amplifying the current signal to obtain an acceleration rate signal.
Certainly, the sensing body can be a beam or a shaft. The present vibration measuring device further includes an actuator connected to the sensing body, and a feedback circuit electrically connected to the piezoelectric sensing layer and the actuator for receiving a measured signal produced by the piezoelectric sensing layer and transmitting a feedback signal to the actuator for controlling a deformation of the sensing structure.
Alternatively the present measuring device can be an active point distributed sensor, and the feedback circuit includes an interface circuit, a feedback compensation circuit and an amplifier, in which the feedback compensation circuit outputs the measured signal proportional to the feedback signal.
Certainly, the piezoelectric sensing layer can be a mode sensor in order that the measuring device serves as an active point distributed sensor of a relatively specified frequency sensitivity, or an anti-mode sensor in order that the measuring device serves as an active point distributed sensor of a relatively specified rejected frequency sensitivity. The present measuring device can be an accelerometer, a device for metering acceleration rate or a force sensor.
Certainly, the feedback circuit can include a proportion/integration/differentiation (PID) controller, and a phase lag compensator or a phase lead compensator. The actuator can be a film actuator made of polyvinylidene fluoride (PVDF) or lead zirconate titanate (PZT), or a point actuator made of polyvinylidene fluoride (PVDF) or lead zirconate titanate (PZT) and driven by an electric field, a magnetic field or an electromagnetic field, or a distributed actuator driven by an electric field, a magnetic field or an electromagnetic field.
According to another aspect of the present invention, a vibration measuring method for measuring a vibration of an object includes steps of providing a sensing body generating a dynamic information in response to the vibration, and providing a piezoelectric sensing layer formed in a distributed mode to measure the dynamic information in a selected specific bandwidth.
Preferably the present vibration measuring method further includes steps of providing an actuator, and providing a feedback circuit for receiving a measured signal produced by the piezoelectric sensing layer and transmitting a feedback signal to the actuator for controlling a deformation of the sensing structure.